Glow-tube for x-ray spectrometry with directly excited samples

ABSTRACT

An improved glow-tube for X-ray spectrometry with directly excited samples, characterized by the positioning of a cold cathode, grid-shaped anode and an opening leading into the spectrometer chamber along the axis of the tube casing. This glow-tube is used for X-ray spectrometric analysis and for structural analysis by diffraction or scattering.

United States Patent 1191 1111 3,857,038

Sahores Dec. 24, 1974 [54] GLOW-TUBE FOR X-RAY SPECTROMETRY 3,118,0641/1964 Attix 313/93 X H DIRECTLY EXCITED SAMPLES 3,373,283 3/1968Lansiart et al 250/389 3,601,612 8/1971 Perez-Mendez.... 250/389 XInventor: Jean Sahor s, Route De Morlaas, 3,681,600 8/1972 Rigden et al250/272 France [731 Assignees Societe Nationale Des Petroles PrimaryExaminer Archie Borchelt D Aqu'tame Pans France Attorney, Agent, orFirm-Brisebois & Kruger [22] Filed: Dec. 26, 1972 [21] Appl. No.:318,198

[57] ABSTRACT [30] Foreign Application Priority Data An improvedglow-tube for X-ray spectrometry with Dec. 29,1971 France 71.47290directly excited samples characterized the pogitioning of a coldcathode, grid-shaped anode and an Cl 250/272, opening leading into thespectrometer chamber along Cl. the axis of the tube casing [58] Fleld ofSearch 250/ This glow-tube is used for X-ray spectrometric analysis andfor structural analysis by diffraction or [56] References CitedScattermg' UNITED STATES PATENTS 6 Claims, 4 Drawing Figures 2,521,6569/1950 Segre et a1. 250/374 PATENTED DEC24 I974 6 I 3k V nia/yeaGLOW-TUBE FOR X-RAY SPECTROMETRY WITH DIRECTLY EXCITED SAMPLES withdirectly excited samples.

Tubes used for X-ray spectrometry include ones which operate in a deepvacuum, of about 10 mm Hg,

' and gas-filled tubes in which the pressure is about to 10 mm Hg.

Vacuum tubes with a heated filament require a deepvacuum installation,and emitting efficiency is low, because of the electrical energyexpended in detaching the electrons from the filament. These fluorescenttubes are either sealed, with a slit, or can be dismantled to allow thesample to be inserted into the vacuum chamber. In both forms, there aredisadvantages :the presence of the slit prevents the passage ofretrodiffused electrons and soft X-photons with a long wavelength,needed to analyse elements with low atomic weight, while the need toplace the sample in a vacuum enclosure means that only materials whichwill not produce degassing can be analysed. The sample, acting as anode,is exposed to intense bombardment by electrons, and can begin to melt,altering its surface composition.

There are also glow-tubes with a cold cathode, emitting a mixture ofelectrons and hard X-rays, and with no slit, which operate in a partialvacuum of 10 to 10' mm Hg, comparable to the vacuum prevailing in v aspectrometry chamber. Exciting of the sample comes from a mixed source,since it receives a simultaneous beam of X-rays and electronsretrodiffused by the anode. During this retrodiffusion, ions are removedfrom the anode metal and deposited on the sample. This metal-coatingeffect is a disadvantage, creating a film which absorbs the X-raysemitted by the sample, and distorting the results of spectrometricanalysis. In addition, at short wavelengths and large amounts of energytransmitted to the sample and needed to excite its characteristic linescould damage substances affected by heat.

The present invention concerns a glow-tube without a slit, whichovercomes these drawbacks, emitting electrons directly, and with highefficiency. More specifically, it concerns a tube containing a cathodeconsisting of a metal disc, and a grid-shaped anode, both electrodesbeing located in the axis of the tube, with an opening on the same axisbehind the anode, through which the electron beam is propagated byinertia, in the direction of the target.

The tube defined in this invention allows a sample to be exciteddirectly, by direct emission of electrons, without any photons. Theenergy spectrum of direct, non-retrodiffused electrons is narrower thanfor retrodiffused electrons, so that energy is saved, since the fluxneeded to excite any line in the sample is much smaller. This ensuresmuch higher emission efficiency, and prevents damage to the sample fromheat. Furthermore, the anode is in the form of a grid, so that the samplc is not exposed to any direct trajectory starting at the wires of theanode grid receiving the electron impacts. This means that the samplewill receive very few anodic ions, and parasitic metallization will beprevented; in addition, the sample no longer diffuses the characteristiclines of the anode metal, which can interfere in the analysis of certainelements. The tube described in the invention ensures high emissionefficiency, up to in contrast to conventional fluorescent tubes (theratio of energy absorbed to energy emitted is approximately 10 Comparedwith existing direct-emission tubes, it offers many advantages,operating without a deep vacuum, in a partial vacuum of approximatelyl0" to 10 mm Hg; and since the sample does not form part of theelectronic optical system, it need not be a conductor.

The use of a grid-shaped anode offers many advantages. Its role is toattract the electrons in the direction of the tube-casing axis, andallow the electron beam to pass through the grid without loss of energy.The electrical field set up between the cathode and anode results onlyfrom the potential between them and, in the absence of any conductingbody in the space through which this beam passes, radiation is perfectlyrectilinear and trajectories are parallel. This means that the surfaceeven of a large'sample is subjected to uniform radiation, preventinglocalized temperature rises that could damage the surface. The electronsare propagated in a parallel beam, so that their velocities are notsubject to localized variations. Since the energy spectrum emitted isnarrow, the exciting efficiency for a characteristic line remains high.

Choice of the transmission coefficient of the grid allows the intensityof the radiation flux to be controlled for a given amount of electricalenergy, notably if there is a possibility of heat damaging the sample.

The following table gives experimental values for different transmissioncoefficients, obtained by selecting appropriate grids. 7

Choice of the surface-area of the grid allows it to be adapted to thesize. of the samples being subjected to radiation, without altering theelectrical. and energy characteristics of the beam emitted;

The tube described in the invention thus allows great versatility in theuse of equipment, which is also dependent on a fixed energy-supplysource.

Type of grid Tube Intensity Temperature flow-rate transmitted of the (at3 kV) sample Nickel wire diam. 0.5 mm

mesh area 0.3

sq. mm 5mA 2.55mA 235C transmission coefficient 50 Steel wire diam. 62.5

mesh area 15 SmA l:l mmA C transmission coefficient 40 accompanying FIG.4 shows a tube according to the present invention attached to a base formounting it in a spectrometer.

FIG. 1 shows, in diagrammatical form, the positioning of the electrodesinside the insulating casing 1. The cathode 2 is a disc or pellet madefrom metal with a high electron-emitting capacity and good resistance toionic erosion, such as aluminium. The anode 3 is a metal-grid made fromrhodium, rhenium or platinum, and connected to earth. The electron beam4 emitted by the cathode travels towards the anode, and is propagated byinertia in the direction of the sample 5 placed in its path. The X-raybeam emitted by the sample, following exciting of the characteristiclines of the elements of which it is formed, is then treated by standardspectrometric methods, using a collimator, crystal and meter, arrangedas a goniometer.

FIG. 2 shows the constructional details of the tube, according to arecommended embodiment of the invention. The casing is in two parts (1aand lb), which slide inside each other. At the end of the inner casingla is a cathode 2 fixed to a threaded rod 6, the length of which can beadjusted and which is held by nuts 7 in the end of the casing. Theelectrical connections 8 are placed inside a flexible insulatingcovering 9. The cathode rod may be attached to a radiator with coolingfins. The outer casing lb has an anode 3 near the end, in the form of agrid connected with earth, with a ring 10, which holds it in a groove onthe inner surface of the casing.

The otuer casing has a capillary inlet pipe 1 1 through which anadjustable flow of gas can be fed in, and the inner casing has an outlet12, so that pressure inside the tube can be kept constant or vary. AnO-ring 13 between the two casings ensures proper sealing. The distancebetween the electrodes can be adjusted by sliding cent tube with theslit 16 in a transverse direction in relation to the axis of the tube 1and its cylindrical base 17. The surface 18 provides a seal duringfitting to a spectrometer.

The design of the tube 1 described in this invention, connected with itsbase 17 by means of a rod 19, as shown in FIG. 4, in a transversedirection in relation to the axis of the cylindrical base 17, allowsthis new tube to be fitted to existing spectrometers without anyalterations to the spectrometer.

The tube described in the invention can be used not only for X-rayspectrometric analysis, but also for structural analysis by diffraction(lattice of a crystalline phase or the structure of a liquid) or byscattering (measurement of the dimensions of particles, mixtures, etc.).

What is claimed is:

1. A glow-tube for use in X-ray spectrometry with directly excitedsamples, said glow-tube comprising at least one elongated insulatingcasing having an opening in one end thereof and containing a disc-shapedcathode and a grid-shaped anode, said cathode and anode being alignedlongitudinally of said casing with said anode between said cathode andopening, said tube being adapted to propagate an electron beam whichpasses through said opening when said cathode is connected to a highvoltage source and said anode is connected to ground.

2. A glow-tube as defined in claim 1, in which said casing comprises twoparts, one of which fits telescopically into the other, with said anodein one of said parts and said cathode in the other, while a sealing ringis positioned between said parts.

3. A glow-tube as defined in claim 1, comprising a capillary tube nearsaid opening for admitting an adjustable flow of gas.

4. A glow-tube as defined in claim 1, carried on an arm attached to acylindrical base, with said casing at right angles to the axis of saidcylindrical base.

5. A glow-tube as claimed in claim 1, in which the said anode is made ofa metal selected from the group consisting of rhodium, rhenium andplatinum.

6. A glow-tube as claimed in claim 1, comprising a control electrodebetween said anode and cathode.

1. A glow-tube for use in X-ray spectrometry with directly excitedsamples, said glow-tube comprising at least one elongated insulatingcasing having an opening in one end thereof and containing a disc-shapedcathode and a grid-shaped anode, said cathode and anode being alignedlongitudinally of said casing with said anode between said cathode andopening, said tube being adapted to propagate an electron beam whichpasses through said opening when said cathode is connected to a highvoltage source and said anode is connected to ground.
 2. A glow-tube asdefined in claim 1, in which said casing comprises two parts, one ofwhich fits telescopically into the other, with said anode in one of saidparts and said cathode in the other, while a sealing ring is positionedbetween said parts.
 3. A glow-tube as defined in claim 1, comprising acapillary tube near said opening for admitting an adjustable flow ofgas.
 4. A glow-tube as defined in claim 1, carried on an arm attached toa cylindrical base, with said casing at right angles to the axis of saidcylindrical base.
 5. A glow-tube as claimed in claim 1, in which thesaid anode is made of a metal selected from the group consisting ofrhodium, rhenium and platinum.
 6. A glow-tube as claimed in claim 1,comprising a control electrode between said anode and cathode.